The present invention relates to a pressure control valve, and, more particularly, to a pressure control valve capable of controlling fluid pressure in response to an electric signal.
Pressure control valves are represented by a diaphragm type pressure control valve, wherein the surface of the diaphragm receives feedback pressure, and a spool type pressure control valve wherein the end surface of the spool receives feedback pressure. The present invention relates to an electromagnetic pressure-control valve of the spool type.
The spool type electromagnetic pressure-control valve is usually designed in such a manner that a load caused by the electromagnet, a spring load, and an output pressure feedback load act on a spool valve which slides in a valve sleeve having a supply port, an output port, and an exhaust port. The output pressure is controlled to a level which corresponds to the electric signal supplied to the electromagnet portion by arranging the balance among the above-described loads (see Japanese Patent Laid-Open No. 60-52509).
The pressure control valve of the type described above is usually operated at a temperature about 150.degree. C. in the case where the valves are included in hydraulic pressure control circuits for automatic transmissions for automobiles. The higher the hydraulic oil temperature becomes, the more hydraulic pressure vibrations are generated, causing pulsations.
Hitherto, the above-described hydraulic pressure vibrations present at high temperatures are prevented by causing a damping effect by having the electromagnet portion constituting the pressure control valve filled with hydraulic oil. That is, even if the plunger tries to vibrate in synchronization with the hydraulic pressure vibrations, the viscous resistance of the enclosed hydraulic oil prevents the plunger from vibrating.
However, the conventional pressure control valve of the type described above causes a problem in that hydraulic oil thus enclosed in the electromagnet portion interrupts the smooth operation of the pressure control valve when the pressure control valve must be operated at low temperature in the case where, for example, the automobile is operated in winter. That is, the viscosity of the hydraulic oil is inevitably raised at low temperature, causing the hydraulic oil to become a viscous fluid acting as a resistance interrupting the movement of the movable portions in the electromagnet portion of the pressure control valve. This leads to a problem in that the pressure governing valve portion deteriorates as regards its designed hydraulic pressure response with respect to the electric signal. As a result, the pressure control valve cannot be reliably operated.